A wetland trafficability hazard index based on soil physical properties and site hydrology evaluations

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Abstract

Harvesting of forested wetlands in the Atlantic and Gulf lower coastal plains has the potential to cause intense site disturbance. Often, as a result of poor pre-harvest planning, silvicultural activities are performed on wetland soils highly susceptible to rutting and puddling. Potential decreases in pine productivity have been connected with increased soil strength and decreased aeration that are commonly remnants of site disturbances associated with wet-weather harvesting. A simple and economical rating system is needed to identify soils susceptible to disturbance by various types of equipment. The use of such a system could lower the impact on wetland soils and lower the cost of extensive site preparation methods. Logging efficiency and operational productivity could also be increased by identifying equipment types compatible with site conditions.
The purpose of this study was to characterize and model soil strength as a function of soil physical properties and site characteristics. The soil strength model was subsequently used to develop a trafficability hazard index. Forested wetland sites in the South Carolina coastal plain were characterized to develop the trafficability hazard index. The study site consisted of three blocks located on poorly drained loblolly pine plantations. Five sequences of measurements were taken consisting of soil moisture, water table depth, and soil strength. The five sequences of measurements were taken over a wide range of soil moisture contents and water table depths to characterize the effects on soil strength. Bulk density, porosity, texture, organic matter, and hydraulic conductivity were also determined to characterize the blocks and identify effects on soil strength.
Evaluating the effects of these properties on soil strength identified two relatively easily determined soil properties that could be used for the trafficability index. Volumetric moisture content and penetration resistance of the A horizon were used due to their relationships with trafficability and ease of determination. Estimates of pressure applied to the soil by harvesting equipment were used to find the limits of the sandy loam A horizon to support various types of equipment. Equipment pressures were compared to soil penetration resistance pressures estimated by soil moisture. The trafficability hazard index presented used general ground pressures for various harvesting equipment, but use of specific equipment pressures would provide the best results. Using the trafficability hazard index, sites with less than optimal conditions for traffic can be avoided or special harvesting equipment can be identified to limit site disturbance.